From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 246ACCCF9F8 for ; Fri, 31 Oct 2025 13:05:07 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vEonI-0005ux-4h; Fri, 31 Oct 2025 09:03:52 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vEonF-0005uI-Lx for qemu-devel@nongnu.org; Fri, 31 Oct 2025 09:03:49 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vEonA-00025g-71 for qemu-devel@nongnu.org; Fri, 31 Oct 2025 09:03:49 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1761915817; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=VTcbbzaSCuE8tzzb+hdM+qI4gRsgcQ5VH+f5vmf7Zjc=; b=dNWE1uVXKsEC3/yZ61PEfwucBXxn9Nt1/xSn+BUN3Bdx60FoZVf1DUcm69l0z3U6WjtGwZ gUy0YSSTrN8DTcQhJCsTJ1j9gp0N/iKefnTbSnPKbjkBq6ia36bWclKo9yb4kae8D/RYRA 6x4VF8IempPby9R0xKIfo7mWR5k4Q4c= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-145-03m2jFz8OKK1QF9NKPoo1Q-1; Fri, 31 Oct 2025 09:03:33 -0400 X-MC-Unique: 03m2jFz8OKK1QF9NKPoo1Q-1 X-Mimecast-MFC-AGG-ID: 03m2jFz8OKK1QF9NKPoo1Q_1761915811 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 845AF18002C9; Fri, 31 Oct 2025 13:03:30 +0000 (UTC) Received: from redhat.com (unknown [10.45.225.146]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 48EB719560A2; Fri, 31 Oct 2025 13:03:26 +0000 (UTC) Date: Fri, 31 Oct 2025 14:03:23 +0100 From: Kevin Wolf To: Hanna Czenczek Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Stefan Hajnoczi , Paolo Bonzini , "Richard W . M . Jones" , Ilya Dryomov , Peter Lieven , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Alex =?iso-8859-1?Q?Benn=E9e?= , Fam Zheng , Ronnie Sahlberg Subject: Re: [PATCH 10/16] qcow2: Fix cache_clean_timer Message-ID: References: <20251028163343.116249-1-hreitz@redhat.com> <20251028163343.116249-11-hreitz@redhat.com> <0d98f477-722c-4023-9b28-54d8faffff66@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0d98f477-722c-4023-9b28-54d8faffff66@redhat.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 31.10.2025 um 10:29 hat Hanna Czenczek geschrieben: > On 29.10.25 21:23, Kevin Wolf wrote: > > Am 28.10.2025 um 17:33 hat Hanna Czenczek geschrieben: > > > The cache-cleaner runs as a timer CB in the BDS AioContext. With > > > multiqueue, it can run concurrently to I/O requests, and because it does > > > not take any lock, this can break concurrent cache accesses, corrupting > > > the image. While the chances of this happening are low, it can be > > > reproduced e.g. by modifying the code to schedule the timer CB every > > > 5 ms (instead of at most once per second) and modifying the last (inner) > > > while loop of qcow2_cache_clean_unused() like so: > > > > > > while (i < c->size && can_clean_entry(c, i)) { > > > for (int j = 0; j < 1000 && can_clean_entry(c, i); j++) { > > > usleep(100); > > > } > > > c->entries[i].offset = 0; > > > c->entries[i].lru_counter = 0; > > > i++; > > > to_clean++; > > > } > > > > > > i.e. making it wait on purpose for the point in time where the cache is > > > in use by something else. > > > > > > The solution chosen for this in this patch is not the best solution, I > > > hope, but I admittedly can’t come up with anything strictly better. > > > > > > We can protect from concurrent cache accesses either by taking the > > > existing s->lock, or we introduce a new (non-coroutine) mutex > > > specifically for cache accesses. I would prefer to avoid the latter so > > > as not to introduce additional (very slight) overhead. > > In theory, the old plan was that eventually qcow2 would use fine grained > > locks instead of the single s->lock, and having a separate cache lock > > would be a step towards it. But if we never actually make use of it to > > hold s->lock for a shorter time, that's not really a good argument. I'm > > not sure if that's ever going to happen unless for a rewrite in Rust or > > something. > > > > I never tried to measure specifically if lock contention is a problem > > with high queue depth and random I/O on a huge disk. Intuitively, > > holding s->lock while doing I/O for loading entries into the cache can't > > be really good. > > > > Anyway, I went a bit on a tangent there... > > > > > Using s->lock, which is a coroutine mutex, however means that we need to > > > take it in a coroutine, so the timer CB must enter such a coroutine. As > > > a result, descheduling the timer is no longer a guarantee that the > > > cache-cleaner will not run, because it may now be yielding in > > > qemu_co_mutex_lock(). > > I think creating a coroutine in cache_clean_timer_cb() is the wrong > > approach. Instead, cache_clean_timer_init() could create a coroutine > > and its implementation could be something like this: > > > > while (!s->cache_clean_timer_stopping) { > > qemu_co_sleep_ns_wakeable(&s->cache_clean_timer_wake, > > QEMU_CLOCK_VIRTUAL, Oh, I wanted to comment on this one, too, but apparently forgot... I have absolutely no idea why we're using QEMU_CLOCK_VIRTUAL in the existing code. I don't think the cache cleaning should stop just because the guest is paused. You can still have block jobs and exports that work on the image and fill the cache. > > s->cache_clean_interval * NANOSECONDS_PER_SECOND); > > > > qemu_co_mutex_lock(&s->lock); > > qcow2_cache_clean_unused(s->l2_table_cache); > > qcow2_cache_clean_unused(s->refcount_block_cache); > > qemu_co_mutex_unlock(&s->lock); > > } > > s->cache_clean_timer_stopping = false; > > Ah, that’s nicer.  I think we can replace the flag by checking > s->cache_clean_interval > 0 and adding a CoQueue to wake up waiting > coroutines. Ah, yes, stopping on s->cache_clean_interval == 0 is good. > > > (Note even now this was only guaranteed for cache_clean_timer_del() > > > callers that run in the BDS (the timer’s) AioContext. For callers > > > running in the main context, the problem may have already existed, > > > though maybe the BQL prevents timers from running in other contexts, I’m > > > not sure.) > > > > > > Polling to await the timer to actually settle seems very complicated for > > > something that’s rather a minor problem, but I can’t come up with any > > > better solution that doesn’t again just overlook potential problems. > > > > > > (One cleaner idea may be to have a generic way to have timers run > > > coroutines, and to await those when descheduling the timer. But while > > > cleaner, it would also be more complicated, and I don’t think worth it > > > at this point.) > > > > > > (Not Cc-ing qemu-stable, as the issue is quite unlikely to be hit, and > > > I’m not too fond of this solution.) > > > > > > Signed-off-by: Hanna Czenczek > > > --- > > > block/qcow2.h | 1 + > > > block/qcow2.c | 90 ++++++++++++++++++++++++++++++++++++++++++++------- > > > 2 files changed, 79 insertions(+), 12 deletions(-) > > > @@ -867,6 +893,39 @@ static void cache_clean_timer_del(BlockDriverState *bs) > > > } > > > } > > > +/* > > > + * Delete the cache clean timer and await any yet running instance. > > > + * Must be called from the main or BDS AioContext, holding s->lock. > > > + */ > > > +static void coroutine_fn > > > +cache_clean_timer_locked_co_del_and_wait(BlockDriverState *bs) > > > +{ > > > + BDRVQcow2State *s = bs->opaque; > > > + IO_OR_GS_CODE(); > > > + cache_clean_timer_del(bs); > > > + if (qatomic_read(&s->cache_clean_running)) { > > > + qemu_co_mutex_unlock(&s->lock); > > > + qatomic_set(&s->cache_clean_polling, true); > > > + BDRV_POLL_WHILE(bs, qatomic_read(&s->cache_clean_running)); > > Polling in a coroutine_fn is verboten. > > > > If we do need this function, I think it would be a yield here and a wake > > on the other side. I think we might be able to get around it if we move > > the call from qcow2_do_open() into qcow2_open() (i.e. outside the > > coroutine). A bit ugly, so your choice. > > We can let a CoQueue do the waking, no? Yes, that's probably nicer. I don't expect that you'll ever have more than a single waiter, but a CoQueue would be safe in either case. > > > + qemu_co_mutex_lock(&s->lock); > > > + } > > > +} > > > + > > > +/* > > > + * Delete the cache clean timer and await any yet running instance. > > > + * Must be called from the main or BDS AioContext without s->lock held. > > > + */ > > > +static void cache_clean_timer_del_and_wait(BlockDriverState *bs) > > > +{ > > > + BDRVQcow2State *s = bs->opaque; > > > + IO_OR_GS_CODE(); > > > + cache_clean_timer_del(bs); > > > + if (qatomic_read(&s->cache_clean_running)) { > > > + qatomic_set(&s->cache_clean_polling, true); > > > + BDRV_POLL_WHILE(bs, qatomic_read(&s->cache_clean_running)); > > > + } > > > +} > > > + > > > static void qcow2_detach_aio_context(BlockDriverState *bs) > > > { > > > cache_clean_timer_del(bs); > > > @@ -1214,12 +1273,20 @@ fail: > > > return ret; > > > } > > > +/* s_locked specifies whether s->lock is held or not */ > > > static void qcow2_update_options_commit(BlockDriverState *bs, > > > - Qcow2ReopenState *r) > > > + Qcow2ReopenState *r, > > > + bool s_locked) > > > { > > > BDRVQcow2State *s = bs->opaque; > > > int i; > > > + if (s_locked) { > > > + cache_clean_timer_locked_co_del_and_wait(bs); > > > + } else { > > > + cache_clean_timer_del_and_wait(bs); > > > + } > > > + > > > if (s->l2_table_cache) { > > > qcow2_cache_destroy(s->l2_table_cache); > > > } > > > @@ -1228,6 +1295,10 @@ static void qcow2_update_options_commit(BlockDriverState *bs, > > > } > > > s->l2_table_cache = r->l2_table_cache; > > > s->refcount_block_cache = r->refcount_block_cache; > > > + > > > + s->cache_clean_interval = r->cache_clean_interval; > > > + cache_clean_timer_init(bs, bdrv_get_aio_context(bs)); > > > + > > > s->l2_slice_size = r->l2_slice_size; > > > s->overlap_check = r->overlap_check; > > > @@ -1239,12 +1310,6 @@ static void qcow2_update_options_commit(BlockDriverState *bs, > > > s->discard_no_unref = r->discard_no_unref; > > > - if (s->cache_clean_interval != r->cache_clean_interval) { > > > - cache_clean_timer_del(bs); > > > - s->cache_clean_interval = r->cache_clean_interval; > > > - cache_clean_timer_init(bs, bdrv_get_aio_context(bs)); > > > - } > > > - > > I think the del/init pair here won't be necessary any more after > > switching to the background coroutine. It will just start using the new > > value of s->cache_clean_interval the next time it sleeps. > > One problem is that if we don’t lock s->lock, the coroutine can read > s->l2_table_cache and s->refcount_block_cache while they’re invalid, which > is why I moved the deletion above. I see. This is a preexisting problem, right? The timer runs in the BDS main context, while qcow2_update_options_commit() runs in the main loop or... essentially anywhere else? Should it be a separate patch then? A comment in the code wouldn't hurt either. > We also still need to delete if the interval is set to 0 (or > special-case that in the coroutine to wait forever). If the coroutine terminates on 0 as you suggested above, that would automatically be solved. > We could run all of this in a coroutine so we can lock s->lock, or we > have to force-stop the timer/coroutine at the start.  Maybe running it > in a coroutine is better… So qcow2_reopen_commit() would immediately enter a coroutine and BDRV_POLL_WHILE() for its completion? It's not exactly pretty (and I hope polling in reopen callbacks is even allowed), but maybe more local ugliness than having additional state (like s->cache_clean_running) to allow BDRV_POLL_WHILE() to wait for the cache clean coroutine to stop. Kevin